The present invention relates to a cap for cooperating with a flow control element of a valve for a self-priming electric pump.
Currently, in the provision of hydraulic systems, the installation of overhead or underhead pumps, depending on the requirements, is widespread.
In the second case, during the pump starting step, the pump aspirates a certain percentage of air and a turbulent circuit is established between the intake region and the delivery region of the impellers. For this reason, it is necessary to provide for the pump a priming system, i.e., a system capable of priming the pump, freeing it from the air.
Self-priming centrifugal electric pumps are currently commercially available which can be installed both in an underhead configuration and in an overhead configuration and in which the self-priming system provides for the presence of a valve that uses a self-priming flow control element.
The self-priming flow control element substantially has the appearance of an elongated body inserted in a duct for recirculation, i.e., for the passage of part of the fluid in output from the set of impellers and diffusers that returns in recirculation to the intake duct. The flow control element usually has, on the side of the intake duct, a head that is surrounded by an O-ring with which it is preset to abut against an adapted shoulder, dividing the two said ducts to then close the passage of fluid in recirculation. A spring is interposed between the head and the opening of the intake duct. On the opposite side, the flow control element rests on a wall of the recirculation duct and therefore can slide, by a distance equal to the length of the spring, along its own longitudinal axis between the shoulder and the wall, in order to close or open the recirculation duct.
The operating principle of these self-priming pumps is as follows. The device draws from the intake duct low-pressure fluid, which is centrifuged in the pump body, acquiring pressure at each stage of the impeller and diffuser assembly, up to the maximum pressure that can be attained at the last of the impellers and diffusers. Part of the fluid, which during the step of activation of the device often contains a certain amount of air, is pushed toward the delivery of the pump, while another part skims the pump body and, through the recirculation duct, returns to the intake duct and therefore returns to circulation together with the intake fluid.
As long as the pump body is crossed by fluid containing air bubbles, the pressure in output is lower than the desired value and the valve remains open, allowing recirculating fluid to flow. These conditions cause a reduction in the performance of the pump, since not all the pumped fluid enters the delivery circuit of the system.
The valve closes automatically, and the pump in this case is termed primed, when in the fluid there is no longer air but only liquid and the difference in pressure between the fluid in output from the last stage of the pump and the fluid at the intake is such as to generate the movement of the flow control element toward the intake duct. The flow control element in fact overcomes the force opposed by the spring and abuts against the shoulder, closing the passage and thus preventing recirculation.
A drop in the delivery pressure causes the movement of the flow control element in the opposite direction and therefore the reopening of the valve and consequent losses that generate a reduction in performance.
This drawback cannot be solved when the self-priming pumps are installed in an underhead configuration, where the level of the water in the tank is higher than the intake level and therefore, since the water enters the pump intake by gravity, there is no air to be expelled with self-priming upon the activation of the device. The pressure variations at the delivery, by causing the movement of the flow control element, and before the opening of the valve, generate useless losses due to the recirculation of the water and noise due to the movement of the flow control element that abuts against the shoulder or against the wall that retains it in the recirculation duct.